Control Method for Direct-Current Transmission by Means of a Plurality of Converters

ABSTRACT

At least three power converters in a power distribution and power transmission system can be controlled as rectifiers or inverters and are connected together by a direct current network. A measuring direct current voltage and a measuring direct current are measured on each power converter and respectively, transmitted to the respective rectifier control and/or inverter control, and a rectifier desired direct power and/or inverter desired direct power is determined for each power converter. The total of all desired direct powers is equal to zero, and a desired direct voltage is determined from each desired direct power, the smallest inverter desired direct voltage of all connected inverters is fixed as minimal direct voltage by means of a minimal direct voltage and the desired direct voltage, a desired direct current is formed from the minimal voltage and the measuring direct voltage, a differential direct voltage is formed from the minimal voltage and the differential direct current is formed from the desired direct current and the measuring direct current. The respective rectifier control of the rectifier is controlled to minimize the total of the differential direct voltage and the differential direct current, and the inverter control of the inverter is controlled to minimize the difference between the differential direct current and the differential direct voltage.

The present invention relates to a method for controlling at least threeconverters, which can be controlled as rectifiers or inverters and areconnected to one another via a DC power supply system, in the field ofpower distribution and transmission.

A method such as this is already known, for example, from DE 195 44 777C1. The method described there is used to control a so-calledhigh-voltage direct-current transmission system, which comprises aplurality of converters, in which case the converter may be selectivelyoperated as inverters or rectifiers. In this case, the converters areconnected to one another via a DC power supply system. Transformers areprovided in order to couple the converters to a respectively associatedpower distribution system. The direct currents and DC voltages arerecorded as measured values at the respective converters in order tocontrol the rectifiers or inverters. Furthermore, nominal value pairs inthe form of a nominal current and nominal voltage are defined for eachconverter. Each control system calculates a control discrepancy relatingto this, that is to say in other words it forms the difference betweenthe measured values and nominal values. The rectifiers are controlledsuch that the sum of the control discrepancies is minimized. Incontrast, the inverters are controlled such that the difference betweenthe control discrepancies is minimized. The already known method has thedisadvantage that it requires a higher control level. However, ahierarchical control structure is complex and can lead to undesirableinstabilities.

The object of the present invention is therefore to provide a method ofthe type mentioned initially, which has a simple structure and at thesame time operates reliably and in a stable manner.

The invention achieves this object by a method in which a respectivemeasured DC voltage and a respective measured direct current aremeasured at each converter and are transmitted to a rectifier controlsystem in order to control the respective rectifier, or to an invertercontrol system in order to control a respectively associated inverter, arespectively associated rectifier nominal DC power and a respectiveinverter nominal DC power are defined for each rectifier control systemand for each inverter control system, with the sum of all the rectifiernominal DC powers and all the inverter nominal DC powers being equal tozero, a respectively associated inverter nominal DC voltage isdetermined from each inverter nominal DC power, the lowest inverternominal DC voltage of all the inverters which are connected to the DCpower supply system is defined as the minimum DC voltage, a rectifiernominal direct current and an inverter nominal direct current arerespectively calculated by means of the minimum DC voltage from eachrectifier nominal DC power and from each inverter nominal DC power, witheach rectifier control system forming the difference between the minimumDC voltage and the respectively received rectifier measured DC voltage,resulting in a rectifier difference DC voltage, and forming thedifference between the respective rectifier nominal current and therespectively received rectifier measured direct current, resulting in arectifier difference direct current, and controlling the associatedrectifier such that the sum of the difference DC voltage and thedifference direct current is minimized, and with each inverter controlsystem forming the difference between the minimum DC voltage and therespectively received inverter measured DC voltage, resulting in aninverter difference DC voltage, and forming the difference between therespective inverter nominal current and the respectively receivedinverter measured direct current, resulting in an inverter differencedirect current, and controlling the respectively associated invertersuch that the difference between the inverter difference direct currentand the inverter difference DC voltage is minimized.

Fundamentally, the invention avoids the need not only for a hierarchicalstructure of the control method but also for control separation.

By way of example, the power nominal values for the control systems forthe converters are defined by a central control point, with the powernominal values being transmitted from the control point via an expedientform of data transmission to the individual control systems. However,the invention avoids the need for a higher-level control system, as hasbecome known from the prior art, actively intervening in the controlprocess in specific, previously defined situations. The control pointjust defines the necessary power nominal values. In fact, instead of thecentral higher-level control system, the lowest inverter nominal DCvoltage is selected and the selected minimum DC voltage is used as thenominal DC voltage for all the control systems for the converters. Thisavoids excessively high DC voltages on the DC voltage side of theconverters, so that the invention avoids the need for a higher-levelcontrol system. The method according to the invention is thereforedecentralized, more dynamic, less complex and more stable than themethods known from the prior art. The lowest inverter nominal DC voltageis preferably selected on a decentralized basis, that is to sayseparately for each control system.

It should be noted that, for example, the measured values are recordedby means of current transformers and/or voltage transformers, whoseoutput signal is in each case proportional to a monitored DC voltage,for example 500 kV, and/or to a direct current, for example 3000 A, thatis produced by this DC voltage. The output signal from the currenttransformer or from the voltage transformer is, finally, sampled by asampling unit in order to produce sample values, and the sample valuesare converted to digital measured values by an analogue/digitalconverter. In other words, the measured DC voltage and the measureddirect current are, for example, digital measured values which aresupplied to the respective control system and are processed further byits software.

Each rectifier and each inverter is advantageously controlled over theentire operating range of the rectifier and, respectively, of theinverter both on the basis of the respectively associated rectifierdifference direct current and on the basis of the rectifier differenceDC voltage and, respectively, on the basis of the respectivelyassociated inverter difference direct current and on the basis of theassociated inverter difference DC voltage. This expedient furtherdevelopment means that control limiting at the sum of or the differencebetween the difference direct current and the difference DC voltage canbe dispensed with. This avoids control separation and even furtherenhances the stability of the method.

The converters are advantageously positioned physically alongside oneanother in order to form a back-to-back link. The back-to-back linkformed in this way is used, for example, in order to couple a pluralityof AC voltage power supply systems.

In one further development of the invention, which differs from this,the converters are positioned at least one kilometer away from oneanother in order to form a long-distance direct-current transmissionsystem. This further development of the method according to theinvention allows electrical power to be transmitted over long distancesbetween more than two converters, as a traditional field of applicationof a direct-current transmission system. In this case, the convertersare generally positioned such that they are separated from one anotherat several hundred kilometers, and are connected to one another via adirect-current link of appropriate length, and are networked and coupledto form a DC power supply system. This allows power transmission betweena plurality of grid points over relatively long distances with lowlosses.

In one exemplary embodiment, a control point transmits the nominal DCpower, as defined by the user of the method according to the invention,with details of the operating mode as a rectifier or inverter to therespectively associated control system for the converters, with eachcontrol system having means to determine the inverter nominal DC voltagefrom the transmitted nominal DC power. Means such as these are, forexample, function transmitters with a characteristic whose profile isdependent on the design and configuration of the respective converter,and of the entire installation, and on empirical values. The invertercontrol systems send the nominal DC voltage determined by them to theother control systems by means of long-distance data transmission.

In the long-distance direct-current transmission process which can becarried out by means of the further development according to theinvention, the respectively required nominal values, such as therespective nominal DC voltage, are interchanged between the convertersby long-distance data transmission means. Expedient long-distance datatransmission means include both cable-based transmission means, such asthe Internet or communication via high-voltage lines, and transmissionmeans without cables, such as radios or the like.

The rectifier and the inverter expediently each have a bridge circuitformed by thyristors. In comparison to other power semiconductor valves,thyristors operate with low losses and are used in particular forhigh-voltage direct-current transmission.

Each rectifier measured direct current, which is normalized with respectto a rated current, is expediently renormalized with respect to therespectively associated rectifier nominal direct current, which islikewise normalized with respect to the rated current, each invertermeasured direct current which has been normalized with respect to therated current is renormalized with respect to the respectivelyassociated inverter nominal direct current, which has likewise beennormalized with respect to the rated current, and both each rectifiermeasured DC voltage and each inverter measured DC voltage which havebeen normalized with respect to the rated voltage are renormalized withrespect to the minimum voltage, with each rectifier difference directcurrent and each inverter difference direct current being calculated asthe difference between unity and the respectively associated rectifiermeasured direct current, which has been renormalized in this way, andthe respective inverter measured direct current, which has beenrenormalized in this way, and with the rectifier difference DC voltageand the inverter difference DC voltage being calculated as thedifference between unity and the respectively associated rectifiermeasured DC voltage, which has been renormalized in this way, and therespective inverter measured DC voltage which has been renormalized inthis way. According to this advantageous further development, the valuesare renormalized while maintaining the required transmission power, thatis to say the nominal DC power. This renormalization process isparticularly advantageous when operating on low loads. Themarginal-current process according to the prior art has a poor controlresponse in the low-load range even in powerful AC voltage power supplysystems, that is to say in AC voltage power supply systems with a highso-called short-circuit ratio, that is to say the ratio of the powersystem short-circuit power to the rated power of the direct-currenttransmission system. By way of example, a high short-circuit ratio is 5.The further development according to the invention in contrast allowsthe desired operating points to be approached quickly even in thelow-load range.

Advantageously, a measured turn-off angle is measured at each inverterand is transmitted to a respectively associated gamma control system,with the gamma control system comparing the measured turn-off angle witha nominal turn-off angle associated with that inverter, and, if therespective measured turn-off angle is less than the associated nominalturn-off angle, producing an inverter DC voltage nominal value which isless than the predetermined inverter nominal DC voltage, with thereduced inverter DC voltage nominal value being transmitted to all theother inverter control systems and all the other rectifier controlsystems, and being used to determine the minimum DC voltage. Accordingto this further development of the invention, a gamma control system isprovided in order to reliably avoid commutation errors when turning onthe converter valves in each inverter. However, in contrast to the priorart, this avoids competitive control using minimum or maximum selectionbetween a gamma control system and, for example, a current controlsystem, for the purposes of the invention. According to the invention,the gamma control system does not operate when the installation to becontrolled is being operated normally. For this purpose, for example, agamma regulator for the gamma control system is locked to the inverternominal DC voltage set by a user of the method. For this purpose, thegamma regulator is limited, for example at the top, to this selectedinverter nominal DC voltage. If the selected nominal turn-off angle isundershot, the gamma control system, in contrast, defines an inverternominal DC voltage which is less than the originally selected inverternominal DC voltage as the inverter DC voltage nominal value, which isthen used to determine the minimum DC voltage. For this purpose, theinverter DC voltage nominal value is advantageously sent to all thecontrol systems. The gamma regulator also expediently has a lowercontrol limit, which ensures that the reduced inverter nominal DCvoltage does not fall below a lower threshold value.

According to a further advantageous further development of theinvention, each rectifier control system has a limiting regulator whichlimits a rectifier regulator in this rectifier control system at the topsuch that a predetermined maximum current and/or a predetermined maximumvoltage are/is not exceeded. The limit, which comes into force forexample in the event of a fault, is used to protect the controlledsystems and for additional stabilization of the method according to theinvention.

According to one expedient further development relating to this, thelimiting regulator limits the associated rectifier regulator when therespectively received rectifier measured direct current is greater thanthe sum of the respective rectifier nominal direct current and apredetermined difference direct-current discrepancy, which is in eachcase associated with the rectifier, or when the respectively receivedrectifier measured DC voltage is greater than the sum of the minimum DCvoltage and a predetermined difference DC voltage discrepancy. Thedifference DC current discrepancy and the difference voltage discrepancymake it possible to set up any desired tolerance band in which adiscrepancy between the respective measured value and the associatednominal value is permissible without infringing the rectifier controlsystem limit as described above.

According to one expedient further development, if the rectifiermeasured DC voltage and/or the inverter measured DC voltage are/isfalling, the rectifier nominal DC power and/or the respective inverternominal DC power are/is reduced as a function of the respectiverectifier measured DC voltage and/or as a function of the respectiveinverter measured DC voltage to a lower value, resulting in arespectively associated fault nominal DC power, with the rectifiernominal direct current and/or respectively the inverter nominal directcurrent being determined from the respective fault nominal DC powerrather than from the rectifier nominal DC power and/or the respectiveinverter nominal DC power. The decrease in the nominal DC power is usedto control the direct-current transmission system in the event of afault in which, for example, a voltage dip occurs in one of the ACvoltage power supply systems or in the DC circuit.

According to one expedient further development relating to this, thefault nominal DC power is defined using a function transmitter which isprovided with a characteristic based on empirical values. In this case,the measured DC voltage is expediently smoothed, and is supplied to thefunction transmitter. The measured DC voltage generally has to besmoothed since the measured DC voltage may fluctuate severely in theevent of a fault. The function transmitter produces a fault limitingpower as a function of the smoothed measured DC voltage. This isexpediently used to limit the output value of an integrator at the top,with the output value of the integrator being the fault nominal DCpower. The output of the integrator is used to determine the nominal DCvoltage and the nominal direct current. During normal operation, theoutput value of the integrator is equal to the nominal DC power selectedby the user, in other words the fault handling process is inactiveduring normal operation. If, in contrast, the respective measured DCvoltage falls below a predetermined threshold value, the functiontransmitter produces a fault limiting power which is lower than thenominal DC power. Initially, this is then the output value of theintegrator, and therefore at the same time the fault nominal DC power.If the smoothed measured DC voltage at the input of the functiontransmitter rises, it produces an increased fault limiting power as theupper limit for the integrator. The integrator then integrates to theincreased fault limiting power, for example at a variable integrationrate. In one preferred exemplary embodiment the integration rate is madedependent on the nature and magnitude of the fluctuation in the measuredDC voltage. In this case, the fluctuation of the measured DC voltage isused as an indication as to whether a given fault is still present orhas already been overcome.

The method according to the invention is suitable not only forhigh-voltage direct-current transmission, and medium-voltagedirect-current transmission but also for low-voltage direct-currenttransmission.

Further expedient refinements and advantages of the invention are thesubject matter of the following description of exemplary embodiments ofthe invention, with reference to the figures in the drawing, in whichthe same reference symbols refer to components having the same effect,and in which:

FIG. 1 shows one exemplary embodiment of the method according to theinvention, on the basis of a high-voltage direct-current long-distancetransmission system having a plurality of converters; and

FIG. 2 shows a detail view of the installation shown in FIG. 1, in orderto illustrate fault limiting in one exemplary embodiment of the methodaccording to the invention.

FIG. 1 illustrates one exemplary embodiment of the method according tothe invention, in the form of a schematic illustration. The figure showsa so-called multiterminal high-voltage direct-current transmission(MT-HVDC) system 1 with a plurality of converters 2, which arecontrolled by the illustrated exemplary embodiment of the methodaccording to the invention. The MT-HVDC system 1 has a power supplysystem connecting transformer 3 for each converter 2, which transformers3 are intended to couple the respective converter 2 to an AC voltagepower supply system 4. In this case, each power supply system connectingtransformer 3 has a primary winding, which is galvanically connected tothe AC voltage power supply system 4 and is inductively connected to twosecondary windings on the power supply system connecting transformer 3.The secondary windings on the power supply system connectingtransformers provide a different phase shift, therefore providing aso-called 12-pulse HVDC system 1 with a plurality of converters 2.12-pulse MT-HVDC systems are very well known by those skilled in the artin this field, so that they do not need to be described in any moredetail at this point.

The secondary windings of the power supply system connectingtransformers 3 are each connected to a bridge circuit composed ofthyristors 5, which are illustrated only schematically in FIG. 1. Bridgecircuits such as these are likewise very well known. A more detaileddescription is therefore likewise superfluous in this case. The bridgecircuit formed from thyristors 5 is controlled in the exemplaryembodiment illustrated in FIG. 1 so as to provide a plurality ofrectifiers 6. The rectifiers 6 are connected via a DC power supplysystem 7 to a plurality of inverters 8, with the DC circuit 7 beinggrounded via resistors 9 to the converters 2. Smoothing inductors 10 areprovided in order to smooth the direct current and are connected in thelink between each converter 2 and the DC power supply system 7. Eachconverter 2 may, of course, be operated both as an inverter and as arectifier.

Each rectifier 6 and each inverter 8 has current transformers which aredesigned to detect a direct current flowing in the rectifier 6associated with it or a direct current flowing to the inverter 8associated with it. At their outputs, the current transformers produce asignal which is proportional to the direct current flowing to therectifier 6 or to the inverter 8. The direct current can be determinedfrom the measurement signal by the use of calibrated appliances. Themeasurement signal is sampled by means of a sampling unit, resulting insample values, and the sample values are digitized by an analog/digitalconverter in order to produce measured direct-current values, with themeasured direct-current values of the rectifiers being referred to asthe rectifier measured direct current Idc_rr and the measured directcurrent values of the inverters being referred to as the invertermeasured direct current Idc_ii.

A measurement signal which is proportional to the DC voltage which isdropped across each rectifier 6 is detected across the resistors 9. Thissignal is also sampled and digitized, resulting in digital measured DCvoltage values, which in this case are referred to respectively as therectifier measured DC voltage Udc_rr and the inverter measured DCvoltage Udc_ii.

In the MT-HVDC system 1, the rectifier 6 and the inverter 8 are severalkilometers apart from one another.

Converter control systems are provided in order to control theconverters 2, with each rectifier having a rectifier control system 11,and each inverter 8 having an inverter control system 12. For clarityreasons, FIG. 1 shows only one rectifier control system 11 and oneinverter control system 12.

A respective rectifier nominal DC power Pdco_r1 . . . Pdco_rr and aninverter nominal DC power Pdco_i1 . . . dco_ii are defined by a controlpoint, which is not illustrated in the figures, for each rectifier 6 andfor each inverter 8, respectively. The nominal DC powers are sent fromthe control point to a radio receiver 13 for each converter 2.

The inverter control system 12 will be described first of all in thefollowing text. In each inverter control system 12, the inverter nominalDC power Pdco_i1 received by the radio receiver 13 is supplied to afunction transmitter 14. The function transmitter 14 has acharacteristic which is used to determine an inverter nominal DC voltageUdcpo_i1 as a function of the received inverter nominal DC power

Pdco_il. The profile of the characteristic of the function transmitteris dependent on the structure, the configuration and the design of theHVDC installation, and is also based on empirical values.

The inverter nominal DC voltage Udcpo_i1 calculated by the functiontransmitter 14 is used as an upper limit for a gamma-PI regulator 15 ofa gamma control system 16 which has means (which are not illustrated inthe figures) for determining a measured turn-off angle γ_(i1) for theassociated inverter 8. Furthermore, the gamma control system 16 has anominal turn-off angle γ₀ _(—) ^(i1), which is applied to the negativeinput of the adder 17 and, in other words, is subtracted from themeasured turn-off angle γ_(i1). The gamma control system 16 has saidgamma-PI regulator 15 and a multiplier 18 in addition to the adder 17.The multiplier 18 is used to define the lower limit for the gammaregulator 15 from the inverter nominal DC voltage Udcpo_i1, withUdcpo_i1 being multiplied by a factor LL_Udco_i1 which is likewisepredetermined. In the illustrated exemplary embodiment the factorLL_Udco_i1 is equal to 0.7. The gamma regulator 15 is accordinglylimited at the top to the inverter nominal DC voltage Udcpo_i1 and atthe bottom to 70% of the inverter nominal DC voltage Udcpo_i1. Duringnormal operation, the gamma control system 16 is inactive, so that theupper limit value Udcpo_i1 is the output value for the integrator 15Udco_i1 at the same time. However, if commutation errors can be expectedas a result of a corresponding measured turn-off angle γ_(i1), thegamma-PI regulator 15 sets an expedient inverter nominal DC voltageUdco_i1, which is sent via a radio transmitter 19 to all the rectifiercontrol systems 11 and to all the other inverter control systems 12.Each rectifier control system 11 as well as each inverter control system12 has a radio receiver 20 for receiving the transmitted inverternominal DC voltages Udco_i2, . . . , Udco_ii from all the inverters 8,or from all the other inverters 8. The inverter nominal DC voltagesUdc_i1 . . . Udc_ii are compared with one another by a minimum selectionunit 21, with the minimum selection unit 21 determining the lowestinverter nominal DC voltage value as the minimum DC voltage Udco. Therest of the control process for the inverter 8 and for the rectifier 6is now carried out on the basis of the minimum voltage Udco, which iscommon to all the control systems.

The minimum DC voltage Udco is used for renormalization of the invertermeasured DC voltage Udc_i1. For this purpose, the inverter measured DCvoltage Udc_i1 and the minimum DC voltage Udco are supplied to a divider22 which divides the inverter measured DC voltage Udc_i1 by the minimumDC voltage. The output of the divider 22 is connected to a negativeinput of an adder 17, with −1 being applied to its second input. Aninverter difference DC voltage du_i1 is calculated in this way. Theinverter difference direct current di_i1 is added to the inverterdifference direct current di_i1 by means of the adder 17.

The process of determining the inverter difference direct current di_i1will be explained in the following text. Each inverter control system 12and each rectifier control system 11 has a limiting device 23 whichcomprises a smoothing unit 24 and a function transmitter 25. Thelimiting unit 23 decreases the originally required inverter nominal DCpower or rectifier nominal DC power Pdco_i1 or the Pdco_r1,respectively, as a function of the respectively measured invertermeasured DC power Udc_i1 or Udc_r1 to Pvdpo_i1 or Pvdpo_r1,respectively. This is expediently done after the collapse of the DCvoltage in the event of a fault, that is to say for example in the eventof a fault in one of the AC voltage power supply systems 4 or elsewithin the DC power supply system 7. Once the fault has been rectified,the DC voltage on the DC voltage link 7 is first of all increased beforethe nominal DC power is raised to the original respective value Pdco_i1or Pdco_r1. The details of the method of operation of the limitingdevice 23 will be described in conjunction with FIG. 2. During normaloperation, the output of the function transmitter 25 Pvdpo_i1 orPvdpo_r1, respectively, is equal to the respectively predeterminedinverter nominal DC power Pdco_i1 or the rectifier nominal DC powerPdco_r1.

The output from the function transmitter 25 is supplied to a divider 26which divides the respective nominal DC power by the minimum DC voltageUdco resulting in an inverter nominal direct current Idco_i1 or arectifier nominal direct current Idco_r1. The inverter measured directcurrent Idc_i1 or the rectifier measured direct current Idc_r1 is thenrenormalized by means of the divider 22, and the inverter differencedirect current di_i1 or, respectively, the rectifier difference directcurrent di_r1 is then determined by the adder 17. The inverterdifference DC voltage du_i1 is subtracted from the inverter differencedirect current di_i1 at the inverter 8. This is done using the equationdi_ii−du_i1=1−x_Idc_i1−1+x_Udc_r1, where x_Idc_i1 and x_Udc_i1 areintended to represent the renormalized measured variables. Thedifference formed in this way is intended to be minimized or, in otherwords, regulated at zero. For this purpose, the output of the adder 17is supplied to an inverter PI regulator 27 which determines the cosineof the trigger angle α at its output. In this case, the inverter PIregulator 27 is limited at the top and bottom to a maximum trigger angleα_(max) and a minimum trigger angle α_(min). The inverter PI regulator27 is followed by an arccosine unit 28, which determines the arccosineand thus the trigger angle α, and supplies them to a trigger generator29, which produces a trigger pulse for the thyristors 5 in the inverter8, as a function of the transmitted trigger angle α.

Each rectifier control system 11 is essentially designed in acorresponding manner to the described inverter control system 12,although the rectifier control system 11 has no gamma regulator 16, and,of course, the rectifier control system 11 does not produce an inverternominal DC voltage, but has said minimum selection unit 21 in order todefine the minimum DC voltage Udco.

Like the inverter control system 12, the rectifier control system 11also has a limiting device 23 and carries out renormalization by meansof the divider 22. However, the adder 17 which precedes the PI DCvoltage regulator 27 does not form the difference between the rectifierdifference current and the rectifier difference voltage but, instead ofthese, the sum of the rectifier difference current and the rectifierdifference voltage, to be precise, after renormalization, using theformula: di_r1+du_r1=1−x_Idc_r1+1−x_Idc_r1.

In contrast to the inverter control system 12, the rectifier PIregulator 27 has a maximum current limit and/or maximum voltage limit.Two adders 30 as well as a minimum selection unit 31 and a PI regulator32 are provided for this purpose. The PI regulator 32 acts on the upperlimit of the rectifier PI regulator 27. The adders 30 add a maximumdifference voltage discrepancy du_xx1 and a maximum difference currentdiscrepancy di_xx1 respectively to the difference DC voltage du_r1 andto the difference direct current di_r1, in each case. If the rectifiermeasured direct current Idc_r1 exceeds a resultant rectifier nominalcurrent value, which is calculated from the sum of the rectifier nominaldirect current Idco_r1 and the maximum difference current discrepancydi_xx1, the rectifier measured direct current is reduced with the aid ofthe PI regulator 32 to the resultant rectifier nominal current value. Ina corresponding manner. The rectifier measured DC voltage Udc_r1 isreduced to a resultant rectifier nominal voltage value, which isobtained from the sum of the rectifier nominal DC voltage Udco_r1 andthe maximum difference voltage discrepancy du_xx1. The greatestdiscrepancy results from the minimum selection unit 31. For thispurpose, the output of the minimum selection unit 31 is supplied to thePI regulator 32 which, at its output, produces a cosine of a controlangle between cos α_(red) _(—) r1 and cos α_(min) _(—) r1. The output ofthe PI regulator 32 is used to limit the PI regulator 27 in therectifier control system 11 at the top. Typical values for du_xx1 anddi_xx1 are between 0.01 and 0.1. Depending on the performance of theHVDC installation, the limit α_(red) _(—) r varies between 40° and 50°.The minimum turn-off angle of the rectifier α_(min) _(—) r1 is normally5°.

At this point, it should be mentioned once again that the values to beadded are, of course, normalized values. In other words, the measuredvalues are normalized with respect to so-called rated values beforerenormalization.

FIG. 2 illustrates the effect of the limiting device 23 in more detailusing the example of a rectifier 6. The rectifier measured DC voltageUdc_r1 is thus supplied to the smoothing unit 24 in order to smooth thevoltage fluctuations, which frequently occur in the event of a voltagedip in one of the AC voltage power supply systems 4, or in the event ofsome other fault, and therefore to covert them to rectifier measured DCvoltages Udc_r1 which can be processed. The smoothed rectifier measuredDC voltage is supplied to the function transmitter 25 together with therectifier nominal DC power Pdco_r1 which has been normalized withrespect to the respective rated value. At its output, the functiontransmitter 25 produces a normalized fault limiting power Pvdpol_r1 onthe basis of a characteristic that is based on the experience of thedesigner of the MT-HVDC system. If the smoothed rectifier measured DCvoltage Udc_r1 exceeds a maximum DC voltage Umax_r1 as a thresholdvalue, the function transmitter 25 produces the rectifier nominal DCpower Pdco_r1, as applied to its input, at its output.

The output of the function transmitter 25 is used for maximum limitingof an integrator 33, with the minimum output voltage of the integrator33 Pmino. Furthermore, a limit-value signaling device 34 with two inputsis provided. The rectifier measured DC voltage Udc_r1 is applied to thefirst input of the limit-value signaling device 34. The maximum voltageUmax_r1 of the function transmitter 25 is fed to the second input. Thelimit-value signaling device 34 compares the two input values. If therectifier measured DC voltage Udc_r1 is greater than the maximum voltageUmax_r1, as is normally the case during rated operation, the output Y ofthe limit-value signaling device 34 is set to be equal to unity. If therectifier measured DC voltage Udc_r1 falls below the maximum voltageUmax_r1 the output of the limit-value signaling device 34 will incontrast be equal to zero. A fault situation therefore results in a zeroas a factor in a multiplier 35, so that the integrator 33 producesvalues Pvdpo_r1 between the minimum power Pmino and the maximum powerPdco_r1 as a function of the drop in the rectifier measured DC voltageUdc_r1. As can be seen from FIG. 1, the rectifier difference directcurrent di_r1 is in this case determined on the basis of Pvdpo_r1.

After fault rectification, the rectifier measured DC voltage Udc_r1rises. The characteristic of the function transmitter 25 results in thisleading to an increase of Pvdpol_r1 at its output. The output of theintegrator 33 is, however, first of all locked at the lowest valuePvdpol_r1 which occurred while the fault existed. However, if thecomparator 34 signals that the rectifier measured DC voltage Udc_r1 isabove a threshold value Umax_r1, the integrator 33 integrates to thevalue Pvdpol_r1 produced by the function transmitter 25. Finally,Pdpo_r1, Pvdpol_r1 and Pdco_r1 match one another, so that a change ismade to normal operation.

The remaining components illustrated in FIG. 2 are used for thecapability to adjust the integration rate of the integrator 33 fromPmino until the rectifier nominal DC power Pdco_r1 is reached. A limiter36 is first of all provided in order to define the integration rate, andchecks whether the rectifier measured DC voltage Udc_r1 is in the rangebetween Umin and WUmin. If Udc_r1 is below Umin, then Umin is producedat the output of the comparator 36, so that a zero signal is produced atthe output of the downstream adder 37, to whose negative input Umin isapplied. The divider 38 therefore likewise produces a zero signal at itsoutput, from which previous voltage values are subtracted by means ofthe adder 39. The previous voltage values between 0 and 1 are producedby the smoothing unit 40 and, in the described situation, are likewisezero.

If, in contrast, the rectifier measured DC voltage Udc_r1 is between thelimits Umin and WUmin, a difference voltage normalized with respect toWUmin is therefore produced at the output of the divider 38. Previoussmoothed voltage values are subtracted from it by means of the adder 39.The value dudt produced at the output of the adder 39 may be positive ornegative, depending on whether the rectifier measured DC voltage Udc_r1is rising or falling. A subsequent minimum selection 41 ensures thatonly negative dudt values are passed on from the minimum selection 41.If the rectifier measured voltage Udc_r1 rises, the product dudt ispositive and the minimum selection 41 passes on a zero to the multiplier42, which multiplies this by the predetermined parameter V_dudt andpasses the resultant product, in this case likewise zero, to the adder43, which then adds this to the likewise predetermined parameterKx_vdpol. The value Kx_vdpol is equal to or greater than unity. If thevoltage is falling, a further minimum selection 44 therefore ensuresthat a value equal to unity is passed to the multiplier 35, whichmultiplies this unity by the output of the limit-value signaling device34 and the likewise preselectable parameter Km_vdpol, and finally makesthis available to the integrator 33. The product Y x km_vdpol×1 is equalto Km_vdpol. The integrator 33 integrates at a selected standard rate.

If the rectifier measured DC voltage Udc_r1 falls during the integrationprocess because of a fault or because of a weak power supply system,dudt is in contrast negative. The dudt value is passed on, is multipliedby V_dudt and is finally added to Kx_vdpol by means of the adder 43, sothat a value of less than unity is produced at the output of the adder43 and is finally passed to the multiplier 35. The integrator 33therefore increases the reduced rectifier nominal DC power Pvdpo_r1 atits output more slowly, using the new time constant determined in thisway.

1-9. (canceled) 10: In a power distribution and transmission system, amethod for controlling at least three converters that can be controlledas rectifiers or as inverters and that are connected to one another viaa DC power supply system, the method which comprises: measuring arespective measured DC voltage and a respective measured direct currentat each converter and transmitting to a rectifier control system inorder to control the respective rectifier, or to an inverter controlsystem in order to control a respectively associated inverter; defininga respectively associated rectifier nominal DC power and a respectiveinverter nominal DC power for each rectifier control system and for eachinverter control system, with a sum of all the rectifier nominal DCpowers and all the inverter nominal DC powers being equal to zero;determining a respectively associated inverter nominal DC voltage fromeach inverter nominal DC power; defining a lowest inverter nominal DCvoltage of all the inverters connected to the DC power supply system asa minimum DC voltage; respectively calculating a rectifier nominaldirect current and an inverter nominal direct current by way of theminimum DC voltage from each rectifier nominal DC power and from eachinverter nominal DC power; with each rectifier control system, forming adifference between the minimum DC voltage and the respectively receivedrectifier measured DC voltage, resulting in a rectifier difference DCvoltage, and forming a difference between the respective rectifiernominal current and the respectively received rectifier measured directcurrent, resulting in a rectifier difference direct current, andcontrolling the associated rectifier to minimize a sum of the differenceDC voltage and the difference direct current; and with each invertercontrol system, forming a difference between the minimum DC voltage andthe respectively received inverter measured DC voltage, resulting in aninverter difference DC voltage, and forming a difference between therespective inverter nominal current and the respectively receivedinverter measured direct current, resulting in an inverter differencedirect current, and controlling the respectively associated inverter tominimize a difference between the inverter difference direct current andthe inverter difference DC voltage. 11: The method according to claim10, which comprises controlling each rectifier and each inverter, overan entire operating range thereof, both on a basis of the respectivelyassociated rectifier difference direct current and on the basis of therectifier difference DC voltage and, respectively, on the basis of therespectively associated inverter difference direct current and on abasis of the associated inverter difference DC voltage. 12: The methodaccording to claim 10, wherein the converters are positioned physicallyalongside one another to form a back-to-back link. 13: The methodaccording to claim 10, wherein the converters are positioned with aspacing distance of at least one kilometer therebetween to form adirect-current transmission system. 14: The method according to claim10, which comprises: renormalizing each rectifier measured in directcurrent, which is normalized with respect to a rated current, withrespect to the associated rectifier nominal direct current, which islikewise normalized with respect to the rated current; renormalizingeach inverter measured direct current, which has been normalized withrespect to the rated current, with respect to the associated inverternominal direct current, which has likewise been normalized with respectto the rated current; and renormalizing both each rectifier measured DCvoltage and each inverter measured DC voltage, which have beennormalized with respect to the rated voltage, with respect to theminimum voltage; calculating each rectifier difference direct currentand each inverter difference direct current as a difference betweenunity and the respectively associated rectifier measured direct current,which has been renormalized in this way, and the respective invertermeasured direct current, which has been renormalized in this way; andcalculating the rectifier difference DC voltage and the inverterdifference DC voltage as a difference between unity and the respectivelyassociated rectifier measured DC voltage, which has been renormalized inthis way, and the respective inverter measured DC voltage, which hasbeen renormalized in this way. 15: The method according to claim 10,which comprises: measuring a turn-off angle at each inverter andtransmitting the measured turn-off angle to a respectively associatedgamma control system; comparing, with the gamma control system, themeasured turn-off angle with a nominal turn-off angle associated withthe inverter, and, if the respective measured turn-off angle is lessthan the associated nominal turn-off angle, producing an inverter DCvoltage nominal value less than the predetermined inverter nominal DCvoltage; and transmitting the reduced inverter DC voltage nominal valueto all the other inverter control systems and all the other rectifiercontrol systems, and using the reduced inverter DC voltage nominal valueto determine the minimum DC voltage. 16: The method according to claim10, wherein each rectifier control system has a limiting regulator forupwardly limiting a rectifier regulator in the rectifier control systemsuch that a predetermined maximum current and/or a predetermined maximumvoltage are not exceeded. 17: The method according to claim 16, whichcomprises setting the limiting regulator to limit the associatedrectifier regulator when the respectively received rectifier measureddirect current is greater than a sum of the respective rectifier nominaldirect current and a predetermined difference direct-currentdiscrepancy, which is in each case associated with the rectifier, orwhen the respectively received rectifier measured DC voltage is greaterthan the sum of the minimum DC voltage and a predetermined difference DCvoltage discrepancy. 18: The method according to claim 10, whichcomprises, if one or both of the rectifier measured DC voltage and theinverter measured DC voltage are falling, reducing the rectifier nominalDC power and/or the respective inverter nominal DC power as a functionof the respective rectifier measured DC voltage and/or as a function ofthe respective inverter measured DC voltage to a lower value, resultingin a respectively associated fault nominal DC power, and determining therectifier nominal direct current and/or respectively the inverternominal direct current from the respective fault nominal DC power ratherthan from the rectifier nominal DC power and/or the respective inverternominal DC power.